1. Field of the invention
The present invention relates to the mounting of turbojet engine units under the wings of aircraft and to nacelles adapted for this purpose.
2. Summary of the prior art
It is known to mount front-fan turbojet by-pass engines under the wings of an aircraft, at the end of a mast suspended from the wing. Such an example is disclosed in French Patent No. 2560 854. This is possible for engines in which the by-pass ratio is between 5 and 10, which results in fan diameters still compatible with a relatively low drag of the nacelle.
Turbojet engine units with very high by-pass ratios have since been made, in which a gas generator drives free contra-rotating turbines which are themselves integral with large, non-ducted contra-rotating propellers. Such engines permit by-pass flow ratios of between 30 and 40 which gives rise to the hope of gains of from 20 to 25% on fuel consumption. The drawback of these engines lies in the large diameter of their propellers which has made it necessary until now to place turbojet engine units of this type laterally on the fuselage of the aircraft, thus restricting their use to a twin-engine formula.
Also under study are engines with very high by-pass ratios (about 15) having single or contra-rotating ducted propellers, which, compared with engines having non-ducted propellers should reduce propeller noise and permit an easier mounting of this type of engine under the wing of an aircraft, as the diameter of the propellers is smaller. Indeed, in practice wing mounting is a necessity for aircraft having more than two engines, such as long range aircraft.
However, in such engines with ducted contra-rotating propellers, the theoretical gain in fuel consumption is almost completely nullified by the correlative increase in the mass and drag of the engine. Indeed, calculations show that heavy carrier aircraft equipped with very high by-pass ratio engines with contra-rotating ducted propellers gain only a few hundred km flying range over a flight of 9000 km as a result of the fact that the actual overall gain in fuel consumption is only 2 or 3% because of the increase in the drag and mass of the engines. For such engines to become competitive it is necessary that substantial improvements should be made on their drag.
Accordingly, it is an object of the present invention to achieve this, while at the same time providing a nacelle structure making it possible to mount such engines under the wings of an aircraft, and no longer only on the sides of the aircraft fuselage.
Another object of the invention is to provide a nacelle having a cowling which facilitates the maintenance of engines of this type, whether they are attached directly under the wing as proposed in one of the embodiments of the invention, or are pod-mounted in a conventional manner, i.e. spaced from the wing, as proposed in an alternative embodiment of the invention.
A further object is to provide a nacelle for these engines including movable flaps for controlling the opening of the secondary nozzle according to flight conditions and incorporating a flow reverser of simple construction.